C++标准库 -- 读写锁的实现

class __shared_mutex_cv
  {
    // Based on Howard Hinnant's reference implementation from N2406.

    // The high bit of _M_state is the write-entered flag which is set to
    // indicate a writer has taken the lock or is queuing to take the lock.
    // The remaining bits are the count of reader locks.
    //
    // To take a reader lock, block on gate1 while the write-entered flag is
    // set or the maximum number of reader locks is held, then increment the
    // reader lock count.
    // To release, decrement the count, then if the write-entered flag is set
    // and the count is zero then signal gate2 to wake a queued writer,
    // otherwise if the maximum number of reader locks was held signal gate1
    // to wake a reader.
    //
    // To take a writer lock, block on gate1 while the write-entered flag is
    // set, then set the write-entered flag to start queueing, then block on
    // gate2 while the number of reader locks is non-zero.
    // To release, unset the write-entered flag and signal gate1 to wake all
    // blocked readers and writers.
    //
    // This means that when no reader locks are held readers and writers get
    // equal priority. When one or more reader locks is held a writer gets
    // priority and no more reader locks can be taken while the writer is
    // queued.

    // Only locked when accessing _M_state or waiting on condition variables.
    mutex		_M_mut;
    // Used to block while write-entered is set or reader count at maximum.
    condition_variable	_M_gate1;
    // Used to block queued writers while reader count is non-zero.
    condition_variable	_M_gate2;
    // The write-entered flag and reader count.
    unsigned		_M_state;

    static constexpr unsigned _S_write_entered
      = 1U << (sizeof(unsigned)*__CHAR_BIT__ - 1);
    static constexpr unsigned _S_max_readers = ~_S_write_entered;

    // Test whether the write-entered flag is set. _M_mut must be locked.
    bool _M_write_entered() const { return _M_state & _S_write_entered; }

    // The number of reader locks currently held. _M_mut must be locked.
    unsigned _M_readers() const { return _M_state & _S_max_readers; }

  public:
    __shared_mutex_cv() : _M_state(0) {}

    ~__shared_mutex_cv()
    {
      __glibcxx_assert( _M_state == 0 );
    }

    __shared_mutex_cv(const __shared_mutex_cv&) = delete;
    __shared_mutex_cv& operator=(const __shared_mutex_cv&) = delete;

    // Exclusive ownership

    void
    lock()
    {
      unique_lock<mutex> __lk(_M_mut);
      // Wait until we can set the write-entered flag.
      _M_gate1.wait(__lk, [=]{ return !_M_write_entered(); });
      _M_state |= _S_write_entered;
      // Then wait until there are no more readers.
      _M_gate2.wait(__lk, [=]{ return _M_readers() == 0; });
    }

    bool
    try_lock()
    {
      unique_lock<mutex> __lk(_M_mut, try_to_lock);
      if (__lk.owns_lock() && _M_state == 0)
	{
	  _M_state = _S_write_entered;
	  return true;
	}
      return false;
    }

    void
    unlock()
    {
      lock_guard<mutex> __lk(_M_mut);
      __glibcxx_assert( _M_write_entered() );
      _M_state = 0;
      // call notify_all() while mutex is held so that another thread can't
      // lock and unlock the mutex then destroy *this before we make the call.
      _M_gate1.notify_all();
    }

    // Shared ownership

    void
    lock_shared()
    {
      unique_lock<mutex> __lk(_M_mut);
      _M_gate1.wait(__lk, [=]{ return _M_state < _S_max_readers; });
      ++_M_state;
    }

    bool
    try_lock_shared()
    {
      unique_lock<mutex> __lk(_M_mut, try_to_lock);
      if (!__lk.owns_lock())
	return false;
      if (_M_state < _S_max_readers)
	{
	  ++_M_state;
	  return true;
	}
      return false;
    }

    void
    unlock_shared()
    {
      lock_guard<mutex> __lk(_M_mut);
      __glibcxx_assert( _M_readers() > 0 );
      auto __prev = _M_state--;
      if (_M_write_entered())
	{
	  // Wake the queued writer if there are no more readers.
	  if (_M_readers() == 0)
	    _M_gate2.notify_one();
	  // No need to notify gate1 because we give priority to the queued
	  // writer, and that writer will eventually notify gate1 after it
	  // clears the write-entered flag.
	}
      else
	{
	  // Wake any thread that was blocked on reader overflow.
	  if (__prev == _S_max_readers)
	    _M_gate1.notify_one();
	}
    }
  };

线程同步的要点：

1.主要变量：互斥锁(mut)，第一道门(gate1)，第二道门(gate2)，读写锁的等待与持有状态(state)。

2.state为unsigned int，去掉最高位后最大值(2**31-1)(约为20亿)，远远大于可能的线程数；最高位记录是否有写线程等待或持有写锁，其他位记录读锁持有数量。mut保护该变量的读写。

3.读线程只需要通过gate1就算持有了读锁；通过gate1的条件是没有写线程等待或已通过gate2(即state最高位为0)并且持有读锁的线程没有达到(2**31-1)(基本不可能)。

4.写线程需要通过两道门才算持有了写锁；通过gate1的条件与读线程相同，在通过后对state最高位置1；通过gate2的条件是没有读线程持有读锁(即state非最高位全部为0)。

5.读线程解锁时：1）如果有写线程等待在gate2，检查是否自己是否是最后一个持有读锁的线程，如果是则唤醒gate2。2）如果没有写线程等待在gate2，检查之前持有读锁的线程数量如果等于(2**31-1)则唤醒gate1(基本不可能)。

6.写线程解锁时由于所有线程均等待在gate1，所以唤醒gate1(notify_all)即可。这里的唤醒非写线程优先，但其会在被唤醒后等待在gate2，阻止后续的读线程通过gate1。